1. Field of Invention
This invention pertains to the art of methods and compounds for providing a thermal barrier coating, and more specifically to methods and compounds for providing a thermal barrier coating with enhanced thermal fatigue life through modification of the bond coat coefficient of thermal expansion in targeted regions of the bond coat microstructure.
2. Description of the Related Art
In the past, thermal barrier coatings included two layer coatings that consisted of a MCrAlX bond coat, wherein M=Ni, Co, or Fe and where X=a reactive element such as Y, Zr, Hf, Yb, or any other reactive element, deposited onto the substrate, and a ceramic insulating layer deposited onto the bond coat. The ceramic insulating layer was typically a zirconia based oxide ceramic, most often a zirconia partially stabilized with 6-8 weight percent yttria. The bond coat was typically 0.005-0.008 of an inch thick and the top coat was typically 0.005-0.020 inch thick. For plasma sprayed thermal barrier coatings, which the present invention specifically addresses, the bond coat was prepared with a rough surface to facilitate adhesion of the outer ceramic layer. The bond coat was required to provide oxidation resistance for the under lying substrate, as well as be oxidation resistant to prevent oxidation failure of the thermal barrier coating. The rough surface enhanced adhesion. The roughness also increased the stresses in the region of the interface between the bond coat and top coat.
The thin, two-layer coating described above works well in current applications. However, these coatings lack the thermal fatigue resistance required for future applications that require longer thermal fatigue life. While numerous modifications have been purposed for new ceramic compositions and structures, few or no proposed approaches address the primary concern for higher durability.
Alternate structures for thermal barrier coatings included "thick" thermal barrier coatings that were developed for diesel engines and outer air seals in aircraft turbines. These thick thermal barrier coats were on the order of 0.080 inch to 0.14 inch thick. The constraint developed by the high thickness of these coatings tended to generate higher thermal stresses than in a thin two-layer coating, causing coating failure of thick coatings at short life times. In order to reduce the stress concentration between the thick ceramic and the metal bond coat, these coatings were modified to "grade" the coefficient of thermal expansion of the coating through the thickness of the ceramic layer. The first layer was 100 percent MCrAlX bond coat layer, while layers two through five contained increasing amounts of yttrium-partially stabilized zirconia and decreasing amounts of MCrAlX. The coatings have also been configured in fewer discrete layers or graded continuously from the bond coat to the top layer. The effect was to gradually alter the coefficient of thermal expansion of the coating from that of the MCrAlX to that of the Yttrium-partially stabilized zirconia, thereby avoiding a high stress concentration within the ceramic layer. The concept worked well for the coatings where there was sufficient distance between the bond coat and top coat to allow for strain isolation. However, such a sufficient distance was not always available.
The disadvantages of thick, graded thermal barrier coatings described above are two-fold. The first is simply that the coatings must be thick to achieve the strain isolation as required. While the added weight of a thick coating is not a serious penalty for all applications, added coating weight is clearly a penalty for applications in the rotating parts in gas turbines, specifically aircraft turbines. The added thickness is also detrimental to the aerodynamics of a turbine blade or vane, especially the trailing edge where a thin total cross section is required. The second disadvantage of thick thermal barriers as they are currently used is that the grading includes certain amounts of isolated MCrAlX within a ZrO.sub.2 --Y.sub.2 --O.sub.3 matrix at some point in the coating. If this layer of the coating is allowed to reach too high a temperature, the MCrAlX will oxidize. Oxidation of the MCrAlX can cause expansion of the ceramic-metallic (cermet) layer to such a degree that the coating is destroyed. For a thin coating for high temperature applications, there is insufficient insulation to protect the graded layer from the high temperature and attendant oxidation that will cause failure. For this reason, thick thermal barriers in high temperature applications require the outer insulating layer to be thick enough to insulate the graded layers. However, this type of coating will then become too thick and too heavy to be used in many applications as described above.
The present invention contemplates a new and improved thermal barrier coating which is simple in design, effective in use, and overcomes the foregoing difficulties and others while providing better and more advantageous overall results.